This invention relates to the field of emergency traffic control equipment. More specifically, it relates to equipment for controlling traffic signals at city intersections so that emergency vehicles can quickly pass the intersection to respond to an emergency call.
Typically in prior art devices for this purpose a radio transmitter is provided in the emergency vehicle. A receiver is provided in the traffic signal light and, upon actuation of the transmitter, the traffic signal is controlled and, for example, a steady red signal in all directions is produced. This situation permits the emergency vehicle to rapidly pass the intersection. Examples of such prior devices are disclosed in the following U.S. Pat. Nos.: 2,903,674 to Schwab; 2,881,409 to Cook et al; 3,209,325 to Mentzer et al; and 3,257,641 to Campana et al.
In some of these devices provision is made for indicating that two emergency vehicles are simultaneously trying to control a traffic signal for the same intersection. In some cases this is indicated by an additional flashing light provided on the signal or the provision of a transmitter in the signal light to transmit a danger signal back to the emergency vehicle. Such methods of indicating a conflict in the control of the signal are not entirely satisfactory. In particular, the standard traffic control signals must be altered by the provision of a transmitter or outfitted with auxiliary beacons or flashing devices.
A further drawback of the cited references is the lack of a security code system to prevent operation of the traffic signals by radio equipment available to the public. Further in an urban environment stray electronic signals, as from paging systems, citizens band radios and the like, have a tendency to inadvertently actuate such systems. The patent to Cook et al partially recognizes this problem and employs a key and control tone signal. It is desirable, however, to provide a positive selectable transmission code system in order that positive control and security are maintained.
It is accordingly an object of the present invention to provide an emergency traffic control system which employs coded transmission, and which activates the traffic control signal only upon reception of the precise selected code.
It is a further object of the present invention to provide a traffic control system which has a positive means of indicating to an emergency vehicle that the intersection which it is desired to control is already under the control of another emergency vehicle or, alternatively, that the intersection is indeed under the control of a first driver.
It is a further object of the present invention to provide a traffic control signal system which does not require the addition of visual signaling devices or mechanical elements but which can be installed in existing traffic control systems.
Other objects and advantages of the invention will become apparent from the remaining portion of the specification.